In the mobile communication system, a reference signal is used for various purposes such as channel estimation, synchronization acquisition, cell search, reception quality measurement and the like. The reference signal is a signal in which values of bits are known in the transmission side and in the reception side before communication, and the reference signal may be referred to as a known signal, a pilot signal, a training signal and the like. It is desirable that the reference signal corresponds one-to-one with a cell ID for identifying a cell. Thus, a large number of reference signals need to be prepared. In the existing system of the Wideband Code Division Multiple Access (W-CDMA) scheme, 512 kinds of reference signals (code sequences) are prepared for the downlink (refer to non-patent document 1, for example).
In the system of the W-CDMA scheme, every reference signal is formed by a random sequence. Correlation between the sequences may become relatively large, but, it dose not come to a large issue since signal quality is ensured mainly by the power in the CDMA based system.
However, in the future mobile communication system planning to perform downlink communication of the OFDM (orthogonal frequency division multiplexing) scheme by using a band wider than that of the present system of the W-CDMA scheme, it is desired to largely suppress other-cell interference. If every reference signal is prepared using the random code sequence like the present system of the W-CDMA scheme, other-cell interference becomes relatively large.
As shown in FIG. 1, from the viewpoint of suppressing interference among cells or among sectors, it is proposed to configure the reference signal by a first sequence formed by a random code sequence and a second sequence which belongs to an orthogonal code sequence group (refer to non-patent document 2, for example). In this proposal, orthogonal code sequences which are different with other are used in a plurality of sectors belonging to the same cell, and random code sequences are used among cells.
FIG. 2 shows relationship among reference signals, cells and sectors. For the sake of simplifying the explanation, although “sectors” are described as a plurality of areas belonging to the same cell, the distinction between “cell” and “sector” is not necessarily strict, and they may be used as equivalent terms as long as there is no fear of confusion. Although three cells and nine sectors in the cells are described as representatives for the sake of simplicity of drawing, similar description applies to other cells and sectors.
In this example, the reference signal is prepared by multiplying a reference sequence by a random code sequence (first sequence) and an orthogonal code sequence (second sequence). A cell identifier (cell ID) for identifying a cell corresponds one-to-one with the reference signal, and the cell identifiers can be prepared by the number of combinations of the random code sequences and the orthogonal code sequences. For example, if 170 random code sequences and 3 orthogonal code sequences are prepared, 510 reference signals and cell IDs can be prepared in total. In the figure, different patterns of the cells correspond to random code sequences which are different with each other for the cells. The symbols “a, b, c” represent 3 orthogonal code sequences which are selected from a orthogonal code sequence group.
Although every cell commonly use a, b and c for its sectors, since random code sequences which are different with each other are used in the cells, the reference signals are different for each sector as a whole. Since sectors included in the same cell are synchronized with each other, interference among sectors can be reduced to substantially 0 by using the orthogonal codes a, b and c. Generally, cell are not synchronized with each other. Thus, interference may remain to some extent. But, since inter-sector interference in the same cell becomes substantially 0, the whole interference amount is small.
FIG. 3 shows a concrete example and a mapping example of the orthogonal code sequence. In the example shown in the figure, a sequence group including three orthogonal code sequences of (1,1,1), (1, exp(j2π/3), exp(j4π/3)) and (1, exp(j4π/3), exp(j2π/3)) is used, so that orthogonalization among three sectors is performed. In the mapping example shown in FIG. 3, a mapping method is devised such that orthogonality can be sufficiently achieved. The reference signal is mapped to time and frequency as shown in the figure, and the reference signal is multiplied by the random code sequence and the orthogonal code sequence. One subframe includes seven symbol durations. The subframe may be referred to as TTI: Transmission Time Interval, and it may be 1.0 ms, for example.
A plurality of frequency components of the reference signal which are transmitted at the same time during a symbol duration have components of phase angles which are different with each other by nθ (integral multiple of θ). Components which are transmitted during different symbol durations in a same subframe include components of phase angles different with each other by (φ+nθ). In the first sector, θ=0 and φ=0 are assigned. In the second sector, θ=exp(j2π/3) and φ=exp(j4π/3) are assigned. In the third sector, θ=exp(j4π/3) and φ=exp(j2π/3) are assigned. When the reference signal is mapped as shown in the figure, every combination of 3 components enclosed by a frame of the case 1, case 2 and case 3 form one orthogonal code sequence.
FIG. 4 shows a mapping example similar to the example of FIG. 3. Moreover, FIG. 4 concretely shows each component c1j (j=1, 2, 3) of the random code sequence and each component (1, exp(j2π/3), exp(j4π/3)) of the orthogonal code sequence to be applied to the reference signal. It is assumed that the random code sequence in the first cell (41 in FIG. 2, for example) is (c11, c12, c13), the random code sequence in the second cell (42 in FIG. 2, for example) is (c21, c22, c23) and that the random code sequence in the third cell (43 in FIG. 2, for example) is (c31, c32, c33). FIG. 4 shows reference signals transmitted in sectors #1, #2 and #3 respectively in the first cell. In any combination of two sectors, the inner product (correlation) among 3 components in the frames of the cases 1, 2 and 3 becomes 0. Therefore, from the viewpoint of enhancing estimation accuracy of the reference signal, it is preferable to use sequences which are orthogonal among sectors as the reference signal.
[Non-patent document 1] 3GPP, TS25.211 “Physical Channels and mapping of transport channels onto physical channels (FDD)”
[Non-patent document 2] 3GPP, R1-062100, NTT DoCoMo, Fujitsu, KDDI, Mitsubishi Electric, NEC, Panasonic, Sharp, Toshiba Corporation, “Orthogonal Reference Signal Design in E-UTRA Downlink”